105 depend ing on the bath formulation substrate type

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105 Depend- ing on the bath formulation, substrate type, contact time, and temperature, CPCCs can be either amor- phous 106 or crystalline. 107 CrPO 4 is often the predomi- nant compound in the coating comprising up to 80% of freshly formed films. 108-110 Most or all of the Cr in CPCC is present in an insoluble Cr 3+ form. Little or no Cr 6+ is present in the coating. As such, these coat- ings do not possess the self-healing characteristics of CCCs and provide corrosion resistance by barrier protection only. Cr 6+ and Cr 6+ Speciation Cr 6+ speciation in coating baths is an important variable in CCC formation. Cr 6+ in solution can speci- ate rapidly via hydrolysis and dimerization to form chromate (CrO 4 2– ), dichromate (Cr 2 O 7 2– ), or bichromate (HCrO 4 ). 4 As described previously, the predominant species present in solution depends on Cr 6+ concen- tration and pH, and not on the form of the salt or acid added to solution. Corrosion-resistant films can be formed over a wide range of pH, suggesting that all three of these species are film-formers to some extent. However, modern chromium-chromate and chromium-phosphate processes are operated at chro- mate concentrations and pH values where dichro- mate is the predominant soluble species. As indicated earlier, solution concentrations of 50 mM Cr 6+ and a pH of 1.6 are typical. Under these condi- tions, the total chromate concentration may vary by a factor of two or more without a significant impact on film formation rate. 111 Activators and Accelerators CCC formation is greatly aided by “activating” species such as fluoride, sulfate, nitrate, formate and acetate, and others. 111 Fluoride is particularly effec- tive in promoting film growth and is often added to both chromium-chromate and chromium-phosphate bath formulations. Fluoride is typically added as NaF and mixed-metal fluoride salts, whose total concen- FIGURE 10. Normalized intensity of the 859 cm –1 Cr 3+ –O–Cr 6+ scattering band as a function of coating time. A representative current vs time trace is shown for comparison. Electrode area was 0.008 cm 2 . Reprinted with permission from The Electrochemical Society.
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CRITICAL REVIEW OF CORROSION SCIENCE AND ENGINEERING 392 C ORROSION—MAY 2003 tration is in the 30-mM to 40-mM range. Fluoride attacks existing surface films, particularly on alumi- num. It readily complexes Al 3+ in solution and has the effect of lowering the Al content in CCC. Conse- quently, CCCs are found to be enriched in Cr, and are of greater thickness when F is added to the bath. 28 Accelerators used in commercial CCC formula- tions include: ferricyanides, acetates, formates, and chlorides, among others. 111 Of these, K 3 Fe(CN) 6 has been most widely studied and described in the litera- ture. K 3 Fe(CN) 6 is added to CCC formulations in 2-mM to 5-mM concentrations to increase coating weight. The active component in this compound is the ferricyanide anion, which increases chromate re- duction reaction kinetics that are normally slow on oxide-covered surfaces. 112 On aluminum, ferricyanide (Fe[CN] 6 3– ) is readily reduced to ferrocyanide (Fe[CN] 6 4– ).
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  • Fall '19
  • pH, Chromates, Chromate conversion coating

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